The present disclosure relates to the field of physical sensing and, more particularly, to the correction of sensor errors due to such factors as nonlinear sensor response and temperature effects, for example.
Control systems operate with electrical signals that represent pressure, force, flow rate, or other sensed physical parameter. The physical parameter must be converted to a current or voltage signal before further processing or analysis by the control system. For example, capacitive and resistive sensors are often used for measuring pressure, and resistive xe2x80x9cbridgesxe2x80x9d are used to measure mass flow. Unfortunately, such sensors typically exhibit nonlinear behavior and temperature dependencies. For example, a pressure sensor may provide a given voltage swing for a given pressure change at a specified temperature and pressure range, but a different voltage swing for the same pressure change over a different pressure range. Or the sensor may provide a different voltage swing for the same pressure change at the same pressure range, but at a different temperature.
Although sensor compensation techniques for such sensor inadequacies as temperature and/or nonlinearity effects are known, they tend to be time consuming and/or costly. A method and apparatus for the compensation of such deficiencies that provides a rapid response, precise compensation, at a relatively low cost would be highly desirable.
A compensated sensor in accordance with the principles of the present invention includes a sensor, a relatively fast feedthrough path, and a relatively slow compensation path. The relatively fast feedthrough path includes a summer and output circuitry, such as a summing amplifier. The relatively slow compensation path includes circuitry that produces one or more correction factors for such sensor deficiencies as temperature dependency, or nonlinearity effects, for example. These one or more correction factors are fed to the summer for summing with the uncompensated sensor output. Additionally, the output of the output circuitry (e.g., summing amplifier), is fed back to the compensation circuitry where it is compared to a compensated sensor output developed by the compensation circuitry. The difference between the compensated sensor signal developed in the compensation circuitry and the output signal fed back to the compensation circuitry is also provided to the summer for summing with the one or more correction factors and the uncompensated sensor output.
In an illustrative embodiment, a compensated sensor in accordance with the principles of the present invention includes a sensor, digital compensation circuitry, and analog circuitry that produces an analog sensor output. An uncompensated sensor output signal is routed to an analog output circuit that sums, in real time, the uncompensated sensor signal with a correction signal developed in the digital compensation circuitry. Using this approach, the analog output signal may respond quickly to changes in the physical parameter being measured by the sensor to produce an updated analog output signal. The digital compensation circuitry may respond more slowly than the analog circuitry to changes in the sensor output, and it operates to xe2x80x9cfine tunexe2x80x9d the analog output signal by adding one or more correction factors, each of which may compensate for one or more sensor deficiencies. The compensated deficiencies may include nonlinear response or temperature dependencies, for example. Additionally, the digital circuitry may compare the compensated sensor output to the analog output and thereby develop an error signal that is summed with the uncompensated signal and the correction signal in real time, by the analog output circuitry, for example. That is, the digital compensation circuitry may be arranged to compensate the raw sensor output and, in a feedback loop, to compare the digitally compensated sensor output to the analog sensor output. The difference between the digitally compensated sensor output and the analog sensor output may then be summed with the digitally compensated sensor output to form the analog sensor output.
In an illustrative embodiment, a compensated sensor includes a sensor, digital compensation circuitry, and analog output circuitry. The digital compensation circuitry functions include: nonlinearity compensation, temperature compensation, zero offsets, and output amplifier compensation. The analog output circuitry responds very quickly to changes in the sensor output so that, for example, the sensor module may be employed in control system applications. The digital compensation circuitry may be less responsive, that is, slower, than the analog output circuitry, but it provides greater precision. The compensated sensor may be particularly useful in a system that requires relatively high-speed response to input signal fluctuations but can accommodate somewhat less rapid output signal compensation. In particular, the compensated sensor may include a pressure or mass flow sensor and may provide feedback to a controller that controls the supply of fluid to a chamber.
A compensated sensor in accordance with the principles of the present invention is particularly well suited to systems that monitor the pressure of a fluid flowing into or out of a chamber. The compensated sensor may be used to monitor the pressure on the inlet or outlet side of the chamber, or may monitor the pressure within the chamber. The compensated sensor""s output may be used to control the flow of fluid into and/or out of the chamber. Similarly, a compensated sensor in accordance with the principles of the present invention may be employed to monitor the flow of a fluid into or out of a chamber and to control such flow.
In an illustrative embodiment, an analog sensor output, S, is fed to a summing amplifier input and to an analog-to-digital converter (ADC). Digital circuitry compensates the output of the ADC for temperature, and/or nonlinearity sensor artifacts that may degrade the sensor""s performance. The digital circuitry may also compensate for other sensor errors. In this illustrative embodiment, the compensation is achieved by adding a correction factor, C1, to the uncompensated signal, S. The analog output, A, (the output of the summing amplifier), is also converted to a digital signal by an ADC. Depending upon design parameters, this may be the same ADC as the one that converts the sensor output to digital form (with the sensor and summing amplifier outputs multiplexed to the input of the ADC) or it may be a second ADC. As is known in the art, each conversion of signals from analog to digital form, and from digital to analog form, may include scaling and offsets to properly represent and compare the converted signals.
The digital circuitry, which may include a microprocessor, performs a comparison to determine the difference between the analog output signal (converted to digital form) and the compensated sensor signal (S+C1). The difference between these signals forms a second compensation factor, CA1. The two compensation factors are used as an error signal. After conversion to analog form C1 and CA1 (separately, or in combination), the error signal is fed to the summing amplifier for combination with the uncompensated sensor output, yielding an input to the summing amplifier of the form S+C1+CA1, where S is the uncompensated sensor output, C1 is one or more compensation factors that compensate for temperature, nonlinearity and/or other sensor deficiencies, and CA1 is a correction factor that compensates for output circuitry error. In this illustrative embodiment, the compensated sensor module also includes a compensated digital output signal. If included, the microprocessor may take on any of several forms known in the art, such as a general purpose microprocessor, a microcontroller, a reduced instruction set controller (RISC), a digital signal processor (DSP), or a core microprocessor of an application specific integrated circuit, for example.